This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Time for more petrels. Having introduced general aspects of petrel biology, diversity and evolution in the previous articles (part I, part II, part III, part IV), it’s now time to get through the different petrel lineages. As explained in the previous article, recent molecular phylogenetic studies indicate that true petrels (Procellariidae) consist of four major clades: Pterodromini (gadfly-petrels), Fulmarini (fulmars, giant petrels and kin), Procellarinii, and Puffinini (shearwaters). The majority of analyses have found Procellarinii and Puffinini to be sister-taxa, with Fulmarini and Pterodromini being successively more distant to this pairing (Bretagnolle et al. 1998, Nunn & Stanley 1998, Kennedy & Page 2002, Penhallurick & Wink 2004). We start with the pterodromines (or pterodromins, if you want).
Pterodromini is a redundant name in phylogenetic terms since it’s currently synonymous with its only included taxon, Pterodroma (though read on). Typically known as the gadfly-petrels, the 35 or so members of this mostly Pacific group are medium-sized petrels that prey on cephalopods, crustaceans and fish. They will dive for these prey (as far down as 5.2 m: Bester et al. 2011) but also seize them while swimming at the surface.
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One of the most famous gadfly-petrels is the Bermuda petrel or Cahow P. cahow. It seems that this species was well known, super-abundant and much exploited during the 1600s, but it then disappeared entirely (being represented in the interim only by bones) until a live one was captured in 1906. This individual was initially misidentified as a Mottled petrel P. inexpectata. A Bermuda petrel nesting colony was famously discovered in 1951 (Murphy & Mowbray 1951). This stuff is recounted in many popular books but less well reported is that the Cahow is now known from the eastern Atlantic: a healthy, live specimen was captured on the Azores in 2002 (Bried & Magalhães 2004) and a possible member of the species was photographed off Denmark in 2011. Genetics shows that the Cahow is especially closely related to Macaronesian gadfly-petrels, with the Black-capped petrel P. hasitata of the western North Atlantic being close to the Bermuda petrel + Macaronesian clade (Jesus et al. 2009).
The Bermuda petrel is strictly protected, nests on four small islets, and seems to be slowly increasing in numbers (1961 = 18 pairs; 2003 = 65 pairs), despite competition for nesting sites with tropicbirds. However, storm surges in 1995 and 1999 completely washed over two of the islets where it nests and caused severe erosion and cliff damage on the other two, destroying 40% of the nesting sites. Increasing sea levels and increasing storminess in the North Atlantic could be catastrophic for this bird (Wingate & Talbot 2003). [Image below uploaded by ZooPro at wikipedia.]
While about 35 gadfly-petrel species are recognised by some authors, this count is by no means universally accepted. Indeed, Harrison (1988) wrote “Few groups engender such fierce arguments as to number of distinct species, even among experts” (p. 237). And perhaps the best known thing about gadfly-petrels is that they are notoriously difficult to distinguish. They mostly look very similar, they mostly fly very quickly, they’re mostly seen in less-than-ideal conditions (at distance, without obvious references for scale), and they’re poorly represented in museum collections, meaning that the key diagnostic features of some forms have not always been that well recorded (Spear et al. 1992). The sexes are alike.
Gadfly-petrels are variably patterned in whites, greys, browns and blacks. Some species are mostly dark (e.g., Kermadec petrel P. neglecta) or entirely dark (e.g., Reunion petrel P. aterrima); others are mostly grey dorsally, mostly white ventrally, and with bold, black markings on the head and wings. Several distinctive field markings are present across the group, including dark caps (e.g., Black-capped petrel, Bermuda petrel), masks (e.g., Hawaiian petrel P. phaeopygia) and distinctive M-shaped markings that extend from wingtip to wingtip across the bird’s dorsal surface (e.g., Cook’s petrel P. cookii, Peale’s or Mottled petrel P. inexpectata). Those ‘M-shaped’ markings are also seen in the prions – is this just a coincidence? White collars (e.g., White-necked petrel P. externa) and underwing patches (e.g., Gould’s petrel P. leucoptera) are present in some gadfly-petrels; throats, chests and bellies are pale in many species (e.g., White-headed petrel P. lessoni, shown below) but dark in others (e.g., Providence or Solander’s petrel P. solandri).
To confuse things further, some of those “almost entirely dark” species include light phase individuals (e.g., Collared petrel P. brevipes, Herald petrel P. heraldica). That is, these species are polymorphic. Or, it has conventionally been thought that they're polymorphic, since - in the case of the Herald petrel - breeding behaviour, acoustics and DNA all suggest that the light phase birds are a different species from the dark phase ones (the light phase birds are P. heraldica sensu stricto, while the dark phase ones warrant recognition as the Henderson petrel P. atrata) (Brooke & Rowe 1996).
In the past, some authors divided up the gadfly-petrels into genera on the basis of size and overall colouration. Classically, Pterodroma was used for the all-dark species, Aestrelata for the large ones with pale bellies, and Cookilaria for the small ones with white bellies. Recent phylogenetic studies do not recover clades that match these tidy subdivisions (there are parts of the cladogram, for example, where certain all-dark species group closer to pale-bellied species than to other all-dark species), but there is a structure nevertheless. Penhallurick & Wink (2004) suggested that the phylogenetic structure they recovered for gadfly-petrels will eventually necessitate the recognition of the names Hallstroma (Kermadec petrel and kin), Proestrelata (Chatham Island petrel P. axillaris and kin) and Cookilaria (Cook’s petrel P. cookii and kin) in addition to Pterodroma sensu stricto.
Incidentally, given that I've been discussing the results of, and citing, Penhallurick & Wink (2004) so extensively (both here and in the previous petrel article), it's only appropriate to note that a critical response to this paper was published by Rheindt & Austin (2005). The latter authors criticised Penhallurick & Wink (2004) for preferring trees without explaining which ones should really be favoured on the basis of branch support, and for playing fast and loose with respect to which populations should be recognised as species (Penhallurick & Wink (2004) lumped together any populations that had a divergence difference of less than 2%). Anyway, required reading for specialists. We move on...
Skua mimicry
I said above that gadfly-petrels mostly prey on crustaceans and cephalopods. Spear & Ainley (1993) wondered why two gadfly-petrels in the Eastern Tropical Pacific – the Kermadec petrel P. neglecta and Herald petrel P. heraldica* – possess conspicuous white inner primaries, a pattern seen elsewhere in skuas. The Kermadec petrel is further skua-like in its bulky body shape and short, squared-off tail. Could it be that these two gadfly-petrels mimic skuas in order to avoid the piratical attentions of skuas themselves (skua generally avoid parasitizing conspecifics), or could it be that they resemble skuas because it proves advantageous for a piratical, skua-mimicking lifestyle?
It seems that both possibilities are at play. None of the more than 350 Kermadec and Herald petrels observed by Spear & Ainley (1993) in their study were attacked by large skuas (like the South Polar skua Catharacta maccormicki), whereas other kinds of petrels were attacked at significantly higher rates (Spear & Ainley 1993). And Kermadec petrels at least seem to use their gross morphological resemblance to skuas to enable successful parasitism of other seabirds. Among their favoured host species were Juan Fernandez petrels P. externa, Tahiti petrels P. rostrata and Wedge-tailed shearwaters Puffinus pacificus. An attack on a tropicbird was also observed.
Kermadec petrels did not exhibit aerobatic behaviour of the sort practised by skuas when they attacked other seabirds and, while skuas approach target birds from just about any trajectory, Kermadec petrels invariably approached targets on the horizontal plane [Kermadec petrel photo uploaded by ZooPro at wikipedia]. Nevertheless, the success rates of these piratical attacks were skua-like in efficacy: that is, Kermadec petrels are about as good at stealing prey from other seabird species as are small skuas. Note that the Kermadec petrel is not as dedicated to parasitism as the skuas it seems to mimic – individuals attacked other birds less than skuas did, and indeed it’s known from other studies that they get most of their food (squid) from non-piratic means (Imber et al. 1995).
* Spear & Ainley (1993) used the name P. arminjoniana for the Pacific gadly-petrels they were studying. The Pacific population was often regarded as a subspecies, P. a. heraldica, but it’s increasingly recognised by petrel experts as the distinct species P. heraldica. If we follow this, the name Herald petrel is best restricted to P. heraldica while P. arminjoniana is best termed the Trinidade petrel.
There’s a lot left to know, and there are lots of things to worry about
I want to finish here by emphasising the point that an enormous amount remains unknown about these attractive petrels. Basic data on the ecology, distribution and life history of many gadfly-petrel species remains unknown. Some are so mysterious that we’re still at the stage where sightings of single individuals are significant enough to put on record.
The Chatham Island petrel, for example, went for 30 years without any record (it was finally rediscovered on Rangatira Island in the Chatham Islands in 1973) and its pelagic range remains almost wholly unknown: Force et al. (2009) reported individuals in the eastern Pacific, off the south coast of Peru. The Vanuatu petrel P. occulta, first collected in 1927 but not named as a new species until 2001, wasn’t observed in living condition until 2006 (Shirihai & Bretagnolle 2010). Round Island in the western Indian Ocean – recently the subject of much interest for reasons related to gadfly-petrel conservation (read on) – was thought to be home to a single breeding gadfly-petrel (the Herald petrel) until the 1980s when Kermadec petrels were discovered to be breeding there. And, in the 1990s, a third species (now identified as P. heraldica) was recorded there as well (Brown et al. 2011).
Given that many seabirds – slow-breeding tubenoses in particular – are threatened by disturbance of their breeding sites, by the predation affects of cats, rats and other introduced predators, by mortality caused by artificial lights, and also by the ingestion of floating plastic and other oceanic debris, the status of some of the rare species are a cause for concern. We know that some species (both Cook’s petrel and Barau’s petrel P. baraui are good examples) have reduced substantially in breeding range due to these factors (Imber et al. 2003, Pinet et al. 2009). As discussed above, we also think that increasing storm surges and climatic instability can cause the loss of breeding colonies (Wingate & Talbot 2003). There is also evidence that habitat change caused by people is leading previously isolated species to hybridise (Brown et al. 2011). This seems to be an increasing problem in the modern world.
Some species have already become extinct as a result of human action. The Large Saint Helena petrel P. rupinarum, named from archaeological remains in 1975, appears to have become extinct some time during or after the 1500s as a result of human hunting, and recently extinct species are also known from the Hawaiian Islands, Canary Islands, Henderson Island and Chatham Island. Serjeantson (2005) documented the former presence of a gadfly-petrel in first millennium Scotland (an area not frequented by any gadfly-petrels today). The species concerned is similar, or identical, to Fea’s petrel P. feae, today restricted as a breeder to the Cape Verde and Madeira Islands (it's one of the Macaronesian gadfly-petrels mentioned above).
With this in mind, and more, it's plausible that we might lose some or many of these obscure seabirds before we ever get to know them.
Not done on petrels yet - more to come. For previous parts in the series, and for other articles on tubenosed seabirds, see...
A symbiotic relationship between sunfish and… albatrosses? Say what?
Living the pelagic life: of oil, enemies, giant eggs and telomeres (petrels part II)
Petrels: some form-function ‘rules’, and pattern and pigmentation (petrels part III)
Putting petrels in their place and the possibly weird evolution of albatrosses (petrels part IV)
And for articles about other kinds of seabirds, see...
To the Sahara in quest of dinosaurs (living and extinct) (includes discussion of gulls and terns)
When bivalves attack (or: bivalves vs birds, the battle continues)
Refs - -
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Bried, J. & Magalhães, M. C. 2004. First Palearctic record of the endangered Bermuda petrel Pterodroma cahow. Bulletin of the British Ornithologists’ Club 124, 202-206.
Brooke, M. de L. & Rowe, G. 1996. Behaviouraland molecular evidence for specific status of lightand dark morphs of the Herald Petrel Pterodroma heraldica. Ibis 138, 420-432.
Force, M. P., Cotton, J. M., Rowlett, R. A. & Balance, L. T. 2009. First records of Chatlam Island petrel Pterodroma axillaris in the eastern Pacific Ocean. Marine Ornithology 37, 277-279.
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